We generally live in harmony with the trillions of little creatures that share our bodies. Thank goodness, since there are so many more of them than us. For every one of your human cells, roughly 100 to 1,000 little bugs live alongside and inside of you. If you were to count all of the cells on and inside of you that are not actually you, in fact, they would number in the tens of trillions, with approximately 1 million such microbes living within every square centimeter of your skin. They mean us no harm, though—as long as everyone follows the rules of engagement. In general, for example, we’d like them to stay out of our brains.

Unfortunately, they rarely follow orders.

Allow me to introduce to you a one-celled parasite known as Toxoplasma gondii. You may have heard about it before because it’s been blamed for various illnesses and cancers, often associated with the handling of cat litter. But Toxoplasma gondii is everywhere; in some parts of the world, up to 95% of a population may be infected with it. Statistically speaking, you and I are both probably infected, but are simply not aware of it.

Now a new field of study call neuroparasitology is being developed to try to understand how these parasites affect our brain function and behavior.

Without doubt, the presence of T gondii in your brain does affect your behavior. Indeed, the evolution of our genus probably owes much to the influence of this parasite in our brains during the past few hundred thousand years. T gondii possesses genes capable of causing our brain to greatly increase its production of the neurotransmitter dopamine. People infected with T gondii, therefore, exhibit many of the symptoms we'd expect of someone whose brain contains far too much dopamine. For example, men infected with T gondii tend to be more extroverted, more aggressive, more suspicious, and more prone to jealousy. In contrast, infected women tend to be more warmhearted and easygoing, and much less prone to jealousy or suspicions. However, infected women both attempt and complete suicide more often than women who do not harbor the parasite in their brains. Not surprisingly, the antipsychotic drugs typically used to block dopamine function are effective against the consequences of the infection.

It is conceivable that this parasite has been living in the brain, and thus influencing our behavior, for as long as the Homo genus has existed. Neuroscientists are speculating whether its presence can predispose someone to schizophrenia—or drive large groups of people living together to go to war. Did possessing this parasite make it more likely that one group of garrulous people triumphed over another, uninfected population?

Clearly, we need to learn more about the subtle balance of forces that exist within our bodies, between all of the creatures who call us home.

© Gary L. Wenk, Ph.D., author of Your Brain on Food Oxford, 2010)

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